On Fri, Nov 09, 2018 at 08:29:53AM +0100, Lukas Wunner wrote:> On Thu, Nov 08, 2018 at 02:01:17PM -0800, Greg Kroah-Hartman wrote:> > On Thu, Nov 08, 2018 at 02:09:17PM -0600, Bjorn Helgaas wrote:> > > I'm having second thoughts about this. One thing I'm uncomfortable> > > with is that sprinkling pci_dev_is_disconnected() around feels ad hoc> > > > I think my stance always has been that this call is not good at all> > because once you call it you never really know if it is still true as> > the device could have been removed right afterward.> > > > So almost any code that relies on it is broken, there is no locking and> > it can and will race and you will loose.> > Hm, to be honest if that's your impression I think you must have missed a> large portion of the discussion we've been having over the past 2 years.> > Please consider reading this LWN article, particularly the "Surprise> removal" section, to get up to speed:> > https://lwn.net/Articles/767885/> > You seem to be assuming that all we care about is the *return value* of> an mmio read. However a transaction to a surprise removed device has> side effects beyond returning all ones, such as a Completion Timeout> which, with thousands of transactions in flight, added up to many seconds> to handle removal of an NVMe array and occasionally caused MCEs.

Again, I still claim this is broken hardware/firmware :)

> It is not an option to just blindly carry out device accesses even though> it is known the device is gone, Completion Timeouts be damned.

I don't disagree with you at all, and your other email is great withsummarizing the issues here.

What I do object to is somehow relying on that function call as knowingthat the device really is present or not. It's a good hint, yes, butdriver authors still have to be able to handle the bad data coming backfrom when the call races with the device being removed.

> However there is more to it than just Completion Timeouts, this is all> detailed in the LWN article.

And that's a great article and your work here is much appreciated. Ithink we are in violent agreement :)